I've heard from renowned astrophysicists that we don't yet know whether or not the Universe is infinite. How is that possible regarding the big bang theory is accepted (as they all do)? Are they referring to the existence of other Universes when they say it could be infinite, or what?

Please clarify: why do you think that Big Bang theory and an infinite universe are somehow incompatible? Are you imagining that an infinite universe is incapable of expanding, perhaps? Or is it something else?
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Stan LiouApr 23 '14 at 2:09

What I mean is, even if it is capable of expanding, if everything originated at the big bang how can that space turn to be infinite in a finite time - the age of the universe.
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harogastonApr 23 '14 at 2:19

I think you're confusing "The Universe" with "The Observable Universe". These are two separate concepts.
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ghoppeJan 14 at 15:18

At 1 second after the big bang, the observable universe and the universe were the same thing. Would you say at this point in time that the universe was "infinite"? If not, what makes the universe different now?
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ScottieJan 14 at 23:50

He's not mistaking "The Universe" with "The Observable Universe" - it's just rather surprising that theoretically universe could have been infinite from the beginning.
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MithoronJan 15 at 23:23

4 Answers
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What I mean is, even if it is capable of expanding, if everything originated at the big bang how can that space turn to be infinite in a finite time - the age of the universe.

In the standard ΛCDM model of the Big Bang, the universe is infinite and has always been such. The Big Bang singularity happened everywhere, in the sense that far back enough in time, the density diverges to infinity at every place.

But this is just a particular model--it assumes that the universe if spatially flat and is globally homogeneous and isotropic. There are extended models in which it is not exactly flat, and so could be finite even if it is still homogeneous and isotropic (if the curvature is even slightly positive). And of course we don't actually know whether it is homogeneous and isotropic at scales much larger than we actually see. Some inflationary models imply that it isn't.

To clarify: the ΛCDM model uses the assumes a spatially flat FLRW solution of general relativity, in which space is the Euclidean $3$-space The Euclidean $3$-space is the only flat homogeneous and isotropic $3$-manifold, so there no way to make it finite without violating at least one of those modeling assumption (e.g., a flat torus could have the same form for the metric, but would not be globally isotropic).

I don't see how this model implies that the universe is infinite. Taken from the wikipedia page you referred to: "The model includes a single originating event, the "Big Bang" [...], which was not an explosion but the abrupt appearance of expanding space-time [...]. This was immediately (within 10−29 seconds) followed by an exponential expansion of space by a scale multiplier of 1027 or more, known as cosmic inflation." "expanding space-time" doesn't seem infinite to me. I'm not implying you are wrong, but I understand a different thing from that article.
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harogastonApr 23 '14 at 2:58

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@harogaston: "expanding space-time" says nothing about either finiteness or infiniteness, but I've edited the answer to point to the specific part of the model that implies the universe is infinite (if we take the model more literally than is justifiable, anyway).
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Stan LiouApr 23 '14 at 3:32

It is known that the universe that we can see in our telescopes is less than the total universe. Since we cannot see what is beyond the visual edge, we cannot determine if the universe is infinite or finite.

Sorry, that doesn't answer my question. Even if we cannot see beyond our observable universe, if everything originated at the big bang how can that space turn to be infinite in a finite time - the age of the universe.
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harogastonApr 23 '14 at 2:14

I think the source of confusion between the two concepts - the Big Bang singularity and an infinite universe - is the misconception that the universe began as a finite expanse originally. This misconception easily arises from analogies using present-day logic and numbers that were not applicable in the early universe. For example, I've heard it said that shortly after the Big Bang, the entire observable universe was the size of a grapefruit, but that explanation neglects to mention that grapefruits would have been much larger then.

The problem is that space is where we can measure how large something is, but space expands, so something that is a certain distance away currently was a lot closer a long time ago, even if neither object has moved in the normal sense. As an analogy to help illustrate the effect:

You and I are standing on a preposterously large deflated balloon. You set down a meter stick, make a mark on the balloon at each end and we each stand on one mark and are now a meter apart. Then I turn on a pump and start inflating the balloon. As the balloon inflates, the surface stretches out and you and I appear to get farther from each other, when though we're not 'moving' (e.g. walking away from each other): now we have conflicting sets of information to consider; according to the marks on the balloon surface we're still one meter apart, but according to the meter stick in your hand (which is not expanding) the distance is greater than that.

Note that while I called the balloon "preposterously large," it could have been infinitely large and still behave the same way. I point this out because I've seen in comments on other answers that you don't see how space could be both infinite and expanding - that if it's expanding, then it must have been previously finite. That is incorrect: in fact, because infinity is the quality of unboundedness, something that is infinitely large can always get bigger, because by definition there is no upper bound on its size.

Note also that if you recorded the earlier analogy in reverse, it would appear that space was shrinking such that a several-meters distance between us reduced over time to one meter. If you continue shrinking the universe in such a manner, it eventually becomes the case that there is zero distance between us. And if you apply that to a scenario where there are people infinitely distributed across the balloon, all of them would come closer together as the balloon deflated, until there was zero distance between any two people... in theory, at least, since real human beings have size. Energy and space don't have size, however, so at the point of the Big Bang, space was still infinite (since an infinite/unbounded space cannot shrink to become finite/bounded) but the distance between any two points in space was zero.

So if you could go back in time to the Big Bang you'd see an infinite ocean of energy, since all the energy was "shoulder-to-shoulder" (infinitely dense) but it rapidly expands (and therefore cools) to the point that basic particles can form, then later matter and molecules. Of course since your size would depend on the metric of space, it wouldn't necessarily look like space was expanding, but simply like the energy and matter were cooling down. In fact we still see this as an effect of spatial expansion in the redshift of light from distant sources: the light "cools down" or loses energy along the way because it is stretched out on its journey through space.

I actually understand the OP's question. If the Universe is 13 billion Years old, then no light can be seen coming from further away than 13 billion light years because nothing existed yet earlier than 13 billion years ago. Then this means that the Universe has to be finite because there is nothing past 13 billion light years away from us.

Actually, this is not true. Because the Universe expands, light is "carried along" with the expansion, and in fact the edge of the observable Universe (i.e. the theoretical limit of what we can observe) is roughly 48 billion lightyears away. The CMB emitted 380.000 after Big Bang, which is the most distant observation we have so far, has traveled a little less, ~46 Gly. The Universe beyond the edge may or may not be infinite, but current data favors an infinite Universe (this has to do with the density of the Universe). If so, is was born infinite.
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pelaJan 14 at 15:14

Remember, that because of the Big Bang, the distances between stars and galaxies is expanding. The size of the observable universe is estimated to be 93 billion light years in diameter.
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ghoppeJan 14 at 15:15

I disagree with your conclusion. Farther than 13 G light-years from us, there is nothing, but there is space, empty. The Universe is infinite, by (my) definition.
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Nicolas BarbulescoJun 29 at 18:35

@NicolasBarbulesco: Sorry, CMB stands for "cosmic microwave background", and is radiation that was released from everywhere in the Universe, in every direction, 380,0000 years after Big Bang, because at this time the temperature had fallen enough that hydrogen turned neutral, emitting the radiation. If the Universe didn't expand, the radiation would travel 1 lightyear per year, but due to the expansion, the distance from the point of emission to a given photon grows faster than this.
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pelaJun 30 at 12:14